Nikola Jevtic Alon Orlitsky Narayana P. Santhanam A lower bound on compression of unknown alphabets. 293-311 2005 332 Theor. Comput. Sci. 1-3 http://dx.doi.org/10.1016/j.tcs.2004.10.038 db/journals/tcs/tcs332.html#JevticOS05